1. Field of the Invention
Embodiments of the present invention relate, in general, to wireless communications and particularly to reducing frame collisions during broadcast and/or multicast transmission in a wireless network.
2. Relevant Background
The minimum unit of a wireless Local Area Network (“LAN”) is referred to as a Basic Service Set (“BSS”). Each BSS typically comprises an Access Point (“AP”) and one or more Stations (“STAs”). Within each BSS the AP periodically performs broadcast transmissions (i.e. a transmission to a non-specific number of receivers without designating the address where data is to be transmitted. A multicast transmission is a transmission of the same message to multiple, designated receivers.) of a beacon frame. Within the beacon, information is provided enabling each of the STAs to synchronize with the AP. Then, after receiving authentication from an AP and formally becoming associated with that AP, it becomes possible to exchange data frames between the AP and each STA. Lastly, in most instances, after receiving data the STA sends to the AP an acknowledgement frame. In addition, each of the STAs within the BSS in an infrastructure network can also communicate with each other STA within the BSS via the AP or in some instances, directly.
There are, as is know by one skilled in the relevant art, two types of methods for controlling the AP in a wireless LAN. One is referred to as a Distributed Coordination Function (“DCF”)/Enhanced Distributed Channel Access (“EDCA”) and the other is a Point Coordination Function (“PCF”)/HCF Controlled Channel Access (“HCCA”). DCF/EDCA is essential in a wireless LAN system while PCF/HCCA is generally considered an optional function.
DCF/EDCA is a communication control system in which each AP/STA within a network has equal transmission rights. In DCF/EDCA, each AP, or the STA to which a transmission is requested, awaits transmission for a period of time; that period of time is determined by adding random-number time, referred to as Random Back Off, to a fixed time, referred to as the Distributed Interframe Space. PCF/HCCA, on the other hand, is a system in which the AP performs transmission control of each STA within the BSS. The AP distributes transmission rights to the STAs in a process referred to as polling. Thus a STA within a BSS in which the AP is using PCF/HCCA can only achieve a frame transmission when it obtains a transmission right from that AP. The interval of frame transmission in the PCF/HCCA is generally defined to be a shorter time period than the Distributed Interframe Space. This shorter time period is referred to as Short Interframe Space.
In IEEE802.11, each parameter for practicing PCF/HCCA/DCF/EDCA is defined but there is no direction as to a preferential use of either DCF or EDCA or PCF or HCCA or, for that matter, the recommended values for each parameter. Details of the actual use depend on the state of the BSS when either DCF/EDCA or PCF/HCCA is actually used.
In a conventional example, consider a multiple access method (multicast) in which packet communication in a wireless communication medium is established between a plurality of terminals and a wireless base station. The applicable system (DCF/EDCA or PCF/HCCA) is selected based on the reception error rate of the packet signals received by the wireless base station. It remains possible in such an example for frames to collide during transmission.
FIGS. 1a, and 1b show two examples of frame collisions that may occur during a broadcast/multicast transmission as known in the prior art. Inner BSS collisions, shown in FIG. 1a represent the scenario when an AP 110 broadcasts a frame to all of its associated STAs 120, 122, 125 and yet at the same time at least one of the STAs 125 transmits a frame back to the AP 110. The two frames collide resulting in a lower Quality of Service for the BSS.
Similarly, FIG. 1b shows two overlapping BSSs (herein referred to as “OBSS”) each having a single AP 130, 140 and their associated STAs 132, 135, 137/152, 155, 157 (respectively). Each AP 130, 140 simultaneously transmits a frame to each of their associated STAs 132, 135, 137/152, 155, 157. STAs 132, 135, 152, 155 which reside in a region exclusively associated with a single transmitting AP receive the frame without difficulty and respond accordingly. However, the STAs residing in 137, 157 overlapping region 160 receive both frame transmissions simultaneously. Furthermore a similar collision can occur when only one AP is broadcasting frames and the other AP and overlapped STA are conducting unicast transmissions which requires the STA to respond.
IEEE standard 802.11 provides an option to minimize or prevent collisions. Current techniques within this area include Hybrid Coordinator Function (“HCF”), Controlled Channel Access, Superframe manipulation, Power Save Multi-polling, and Mesh Deterministic Access. Unfortunately current techniques including those listed above fail to prevent intra BSS and inter BSS frame collision. Embodiments of the present invention address these and other deficiencies of the prior art.